A lens is an optical device which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element, while a compound lens is an array of simple lenses with a common axis. Lenses are used as prosthetics for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism. Many lenses used for other purposes have strict axial symmetry, whereas eyeglass lenses are only approximately symmetric. Lenses are also used in imaging systems, such as monoculars, binoculars, telescopes, microscopes, cameras, and projectors. Some of these instruments produce a virtual image when applied to the human eye, while others produce a real image which can be captured on photographic film or an optical sensor, or can be viewed on a screen. In these devices, lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror.